The United States maintains huge stockpiles of plutonium and highly enriched uranium (HEU) for weapons use—far larger than needed for any conceivable military contingency. After disposing of the plutonium it has designated as excess to military requirements, the United States will retain enough for some 10,000 nuclear weapons. After disposing of HEU it has deemed excess, the United States will have enough for 10,000 to 16,000 nuclear weapons. Though still grossly oversized, the US nuclear arsenal has dropped to somewhat fewer than 5,000 weapons, including those in reserve. The United States should declare all fissile material above the amount needed for 5,000 weapons as excess to military needs and dispose of it expeditiously. To reduce the risk that terrorists could steal and use US plutonium to make a nuclear weapon, the United States should directly dispose of excess plutonium rather than using it to fuel reactors.